Method of forming a protective oxide ceramic coating on the surface of valve metals and alloys

ABSTRACT

The invention relates to the field of electroplating. A method of plasma electrolytic oxidation (PEO) includes submerging an article as an electrode, together with a counter electrode, in a bath filled with an aqueous alkaline electrolyte, and supplying bipolar rectangular voltage pulses to the electrodes using a pulsed power supply. The ratio of the duration of an anodic pulse to dead time is selected in a range of 1:5-1:6, the duration of an anodic pulse is 3-30 μs, and the total duration of a period is 30-300 μs. PEO is carried out at the following amplitude values: 600-1200 V for the anodic voltage pulses, and 150-400 V for the cathodic voltage pulses. The invention provides for the production of fully melted, homogeneous oxide ceramic coatings having a high microhardness, a high elastic coefficient, high adhesive and cohesive strength, and a high density.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a National stage application from PCT application PCT/RU2019/000089 filed on Feb. 13, 2019 which claims priority to Russian patent application RU2018101685 filed Jan. 17, 2018, all of which are incorporated herein by their entirety.

FIELD OF THE INVENTION

The present invention relates to the field of protective-coating deposition and particularly to the plasma electrolytic oxidation (PEO) of articles made of valve metals and alloys. By using the method disclosed herein, oxide ceramic coatings having an improved wear resistance, corrosion resistance, thermal resistance, and dielectric strength are formed on the surface of these articles for relatively short time. The method of forming the coatings according to the present invention may be used in the mechanical-engineering, automotive, aerospace and radio-electronic industries, and medicine, both in one-off production and in batch production.

BACKGROUND OF THE INVENTION

PEO methods involving high-frequency pulse electrolysis modes and a range of high polarization voltages is a new development in the PEO technology.

WO 03/83181 discloses a method and apparatus for forming ceramic coatings on metals and alloys. The method allows the ceramic coatings to be formed on valve metals and alloys by using a current pulse rate from 500 to 10000 Hz. The drawback of the method is a waveform of anodic and cathodic current pulses supplied to electrodes, which has a spike at a leading edge. Such a waveform of the current pulses may lead to the occurrence of overcurrent in crucial power electronic elements of high-frequency transducers and makes it hard to choose these expansive elements correctly.

US 20160186352 discloses a non-metallic coating and a method of its production. According to the method, bipolar voltage and current pulses having a trapezoidal waveform and a pulse rate 0.1-20 kHz are supplied to electrodes. An amplitude of anodic voltage pulses is maintained in a potentiostatic mode, while an amplitude of cathodic current pulses is maintained in a galvanostatic mode. The drawback of the method is that it involves using the voltage and current pulses with the trapezoidal waveform, which are less energy-efficient than pulses with a square waveform, thereby adversely impacting the performance of the PEO process. Furthermore, during the oxidation in these modes and when sufficiently high thicknesses of a ceramic coating are achieved under the conditions of ever-increasing cathodic voltage and constant cathodic current, the power of the cathodic pulses may increase so much that it leads to the degradation of the already-formed coating.

WO 2008120046 discloses a method of forming a protective ceramic coating on the surface of metallic articles. The method involves using short high-power voltage pulses having a square waveform, with a duration of anodic pulses being 5-20 microseconds, and a ratio of durations of anodic and cathodic pulses being equal to Ta/Tc=0.2-0.4.

The main drawback of the inventions disclosed in US 20160186352 and WO 2008120046 is an absence of sufficiently long dead time within the period after the propagation of the anodic voltage and current pulses and before the cathodic voltage and current pulses. Such dead time is required to correct and restore concentrating and thermal conditions in near-electrode electrolyte layers.

In the above-indicated patent documents, the bipolar voltage and current pulses are followed by each other with no dead time therebetween. In case of high pulse powers, a local discharge energy takes on values such that resultant microvolumes of a molten oxide have no time to be fully crystallized, for which reason a subsequent discharge occurs at the same place. This adversely impacts the stability of the PEO process and the quality of the coating being formed.

SUMMARY OF THE INVENTION

The main objective of the invention is to form, by using the method disclosed herein, an oxide ceramic coating having physical-mechanical and protective properties higher than those of the coatings formed by using the prior art PEO methods. This involves a higher microhardness, elastic coefficient, adhesive and cohesive strength, and density of coatings. The improvement of the physical-mechanical properties leads to improving practical protective characteristics of coatings, such as an abrasive and erosive wear resistance, a resistance to vibration and cavitation loads, a corrosion resistance. In turn, the improvement of the practical protective characteristics of the coatings allows increasing operational characteristics of articles with the coatings, as well as significantly extending the range of use thereof.

Another objective of the invention is to provide the possibility of performing the PEO process with high speeds of oxide-ceramic coating formation by using short high-power (voltage and current) pulses, while avoiding undesired microplasma-to-arc discharge transitions and coating “loosening”. The oxidation process in intensive electrical modes allows not only increasing the process performance, but also obtaining the enhanced melted oxide ceramic coatings.

To achieve the objectives above, the invention provides a method of forming a protective oxide ceramic coating on the surface of articles made of valve metals (aluminum, titanium, magnesium, zirconium, tantalum, niobium, beryllium) and alloys thereof, in which an article as an electrode is submerged together with a counter electrode in a bath filled with an aqueous alkaline electrolyte, and bipolar voltage pulses providing the implementation of the process in the PEO mode are supplied to the electrodes.

The novelty is provided by that

-   -   bipolar square voltage pulses are supplied to the electrodes, an         anodic pulse being followed by dead time and then a cathodic         pulse, wherein a ratio of durations of the anodic pulse and the         dead time (Ta/To) is selected in a range of from 1:5 to 1:6, the         duration of the anodic pulse is 3-30 microsec, and a total         duration of a period (T=Ta+To+Tc) is 30-300 microsec;     -   during the oxidation, a relationship Ua*Ta=Uc*Tc is observed         between amplitude values and durations of the anodic and         cathodic voltage pulses.

PEO process conditions (the amplitudes and durations of the anodic and cathodic voltage pulses, an effective current density in anodic and cathodic circuits, an oxidation process duration) significantly impact both the process performance and coating quality, i.e. the physical-mechanical characteristics of the oxide ceramic coating.

The invention describes a high-voltage high-frequency anode-cathode PEO method, which is a new promising development in the PEO methods. Processes of electroplasma-chemical reactions significantly speed up under the conditions of an extremely small pulse duration, a high pulse rate and high voltage pulse amplitudes. Moreover, an ion migration speed rapidly increases in breakdown areas. All of this causes a substantial increase in the speed of oxide-ceramic coating formation.

This invention involves modes in which an anodic voltage pulse is followed by quite long dead time and then a cathodic current pulse. In the meantime, the ratio of durations of the anodic pulse and the dead time (Ta/To) is selected in a range of from 1:5 to 1:6, the duration of the anodic pulse is 3-30 microsec, and the total duration of the period (T=Ta+To+Tc) is 30-300 microsec. Such dead time is required to equalize the concentration of an electrolyte in a near-electrode space and to absorb the heat resulted from discharges and electroplasma-chemical reactions by a metal substrate and the electrolyte. These processes occur due to the convection, diffusion and interaction of electrolyte ions to each other.

The dead time has a minimum duration calculated based on the following ratio of the durations of the anodic pulse and the dead time: 1:5. It is the lowest required time for the relaxation and stabilization of a microplasma-breakdown process. A significant increase in the dead time will lead to a reduction of the PEO process performance.

The above-indicated optimal duration of the period, i.e. 30-300 microseconds, corresponds to the pulse rate 3.3-33 kHz, with the duration of the anodic pulses being 3-30 microseconds.

The authors of the invention have experimentally determined the following optimal relationship between the amplitude values and durations of the anodic and cathodic voltage pulses: Ua*Ta=Uc*Tc. This relationship is observed by using a microprocessor system for controlling a pulsed power supply. Such a relationship between the pulse voltages and durations, together with the dead time, provides the formation of hard, strong and dense oxide ceramic coatings.

The high speeds of oxide-ceramic coating formation are provided by using short high-power (voltage and current) pulses, assuming the avoidance of undesired microplasma-to-arc discharge transitions and coating “loosening”.

The largest thickness of the coatings for relatively short oxidation time is achieved at high amplitude values of pulse voltages.

Optimal thicknesses of the protective ceramic coatings are 20-100 microns, depending on a task to be solved and a material to be processed. These thicknesses are achieved for the oxidation time 5-20 minutes. To form such coatings in aqueous alkaline electrolytes, the processing is performed at the following voltages: anodic voltages 600-1200 V and cathodic voltages 150-450 V, depending on the nature of materials to be processed. When processing aluminum and its alloys, the amplitude of the anodic voltage is 900-1200 V, and the amplitude of the cathodic voltage is 250-450 V. When processing titanium, magnesium, tantalum, zirconium, niobium, beryllium and their alloys, the amplitude of the anodic voltage is 600-800 V, and the amplitude of the cathodic voltage is 150-250 V. High pulse voltages lead to an increase in the penetration depth of mictoplasma breakdowns (almost up to the metal substrate), which causes the formation of the coatings with a uniform composition and structure across the whole coating thickness.

A voltage level is closely related to the effective current density. At high voltage amplitudes, the main electrolysis parameters controlled in the PEO process are represented by the value of the effective current density (or average current) in the anodic and cathodic circuits. The current density impacts the number of microplasma discharges and, consequently, the process performance and the melting level of the coatings. The oxidation process is carried out at the effective current density 5-20 A/dm2 in the anodic period and 6-25 A/dm2 in the cathodic period, depending on the nature of the material to be processed. At the effective current densities lower than the optimal values, the hardness of the coatings is decreased, as well as oxidation performance is reduced. At the effective current densities higher than the optimal values, sizes of crystals in the coatings are increased and a coating porosity gets high, for which reason a coating strength and density is reduced.

The technical result of the present invention consists in forming, by using the above-described intensive electrolysis modes, fully melted, homogeneous oxide ceramic coatings having a uniform thickness and unique physical-mechanical properties: a high hardness, elastic coefficient, adhesive and cohesive strength, and density.

In the method disclosed herein, depending on oxidation conditions and desired results, the oxidation process is carried out in a pulsed potentiostatic or pulsed galvanostatic mode within the anodic period, and within the cathodic period—in a pulsed potentiodynamic mode with a uniform rate of increase of the amplitude of the cathodic voltage pulses 1-3 V/min or in a pulsed galvanodynamic mode with a uniform rate of decrease of the amplitude of the cathodic current pulses 0.2-0.5 A/min.

Thus, during cathodic polarization, the slow rate of increase of the amplitude values of the cathodic voltage pulses is compensated by the slow rate of decrease of the amplitude values of the cathodic current pulses and, correspondingly, an average cathodic current.

For example, given that the oxidation is applied to a variable load in the bath (i.e. a different number of members and their shapes), the PEO process is carried out in the pulsed galvanostatic mode within the anodic period and in the pulsed galvanodynamic mode within the cathodic period in order to increase coating quality reproducibility. Given a stable high load in the bath (which corresponds to batch production), the PEO process is carried out in the pulsed potentiostatic mode within the anodic period and in the pulsed potentiodynamic mode within the cathodic period.

The invention is illustrated by the following exemplary embodiment of the method.

As samples (5 pieces), disks of 61 mm in diameter and 5 mm in thickness, made of heat resistant aluminum alloy AK4-1 (2618 T6), were used. During oxidation, the disk is submerged, together with two counter electrodes made of stainless steel, in the bath with a silicate-alkaline electrolyte having pH 10 (the composition of the aqueous alkaline electrolyte will be different for other valve metals).

Square voltage pulses with pulse rate 5.7 kHz were suppled to the electrodes. The duration of the anodic pulses was 15 microsec, the duration of the cathodic pulses was 65 microsec, and the duration of the dead time between the anodic and cathodic pulses was 95 microsec. The amplitude of the anodic voltage pulses was 1200 V, and the amplitude of the cathodic voltage pulses was 250-280 V. The effective current density was 14 A/dm2 in the anodic circuit and 18-16 A/dm2 in the cathodic circuit. The oxidation time was 19 min, and the thickness of the formed coating was 80 microns.

The study of the oxide ceramic coatings on the samples was carried out by using the state-of-the-art measurement equipment. The hardness and elastic coefficient of the coatings were measured on microspecimens by using a nano-hardness tester (available from CSM Instruments) with load 20 mH. The hardness was 25-30 GPa and the elastic coefficient was 330-350 GPa across the whole section of the coating (from the external layer to the metal substrate).

The adhesive and cohesive strength of the coatings was measured by using adhesion tester Revetest (available from CSM Instruments). The results of scratch testing were used to calculate the adhesive and cohesive strength of the coatings—it was equal to 300-320 MPa. The porosity of the coatings was determined on the microspecimens by using scanning-electron microscope S-3400N (available from Hitachi) with image resolution 3 nm. Pore sizes (diameters) in the coating were within in a range of 90 to 200 nm. The wear resistance of the coatings was estimated on a tribometer (available from CSM Instruments) at sliding friction according to the “ball-disk” scheme (a sliding distance was 2500 m). The average wear of the samples was 0.7*10-7 mm3/H/m.

The study of the coatings formed by using the method disclosed herein has shown that they outperform the coatings formed by using the prior art methods in terms of their physical-mechanical properties, namely: in terms of the hardness, elastic coefficient and strength by 1.5 time, and in terms of the pore size and density by 1.3 time. This ensures a significant improvement of operational characteristics of articles with the oxide ceramic coatings formed by using the method disclosed herein.

The method disclosed herein was used to produce stages (impellers and diffusers) of multistage electric-centrifugal submersible crude-oil pumps from B95 aluminum alloy with a protective nanostructured oxide ceramic coating. The pumps equipped with these new light stages were subjected to downhole testing under the conditions of pumping over a corrosive abrasive-carrying oily mixture. The results of the downhole testing have shown a threefold extension of lifetime compared to pumps with standard stages made of nickel cast iron—Ni-Resist. 

What is claimed is:
 1. A method of forming a protective oxide ceramic coating on the surface of articles made of valve metals and alloys by using plasma electrolytic oxidation (PEO), wherein the method comprises: submerging an article as an electrode, together with a counter electrode, in a bath filled with an aqueous alkaline electrolyte, and supplying, by a pulsed power supply, bipolar voltage pulses to the electrodes, wherein the method is characterized in that bipolar square voltage pulses are supplied to the electrodes, an anodic pulse being followed by dead time and then a cathodic pulse, wherein a ratio of durations of the anodic pulse and the dead time (Ta/To) is selected in a range of from 1:5 to 1:6, the duration of the anodic pulse is 3-30 microsec, and a total duration of a period (T=Ta+To+Tc) is 30-300 microsec; a relationship Ua*Ta=Uc*Tc is observed between amplitude values and durations of the anodic and cathodic voltage pulses, and the PEO is performed at the following amplitude values of the voltage pulses: anodic voltage pulses 600-1200 V and cathodic voltage pulses 150-450 V, depending on the nature of a material to be oxidized.
 2. The method of claim 1, wherein the protective oxide ceramic coating is formed on metals and aluminum, titanium, magnesium, zirconium, tantalum, niobium, beryllium alloys.
 3. The method of claim 1, wherein the PEO of aluminum and its alloys is performed at the amplitude values of the anodic voltage pulses 900-1200 V and the cathodic voltage pulses 250-400 V, and the PEO of titanium, magnesium, zirconium, tantalum, niobium, beryllium and their alloys is performed at the amplitude values of the anodic voltage pulses 600-800 V and the cathodic voltage pulses 150-200 V.
 4. The method of claim 1, wherein the PEO is performed at effective current densities 5-20 A/dm² in an anodic circuit and 6-25 A/dm² in a cathodic circuit, depending on the nature of the material to be processed.
 5. The method of claim 4, wherein the PEO is performed in a pulsed potentiostatic or pulsed galvanostatic mode in the anodic circuit, and in the cathodic circuit—in a pulsed potentiodynamic mode with a uniform rate of increase of the amplitude of the cathodic voltage pulses 1-3 V/min or in a pulsed galvanodynamic mode with a uniform rate of decrease of the amplitude of the cathodic current pulses 0.2-0.5 A/min. 